The Air Force launched the unmanned spacecraft Tuesday aboard an Atlas V rocket.

It's the second flight for this original X-37B spaceplane. It circled the planet for seven months in 2010. A second X-37B spacecraft spent more than a year in orbit.

These mystery machines are about one-quarter the size of NASA's old space shuttles and they can land automatically on a runway.

The military isn't saying much if anything about this new secret mission. But one scientific observer, Harvard University's Jonathan McDowell of the Harvard-Smithsonian Center for Astrophysics, speculates the spaceplane is carrying sensors designed for spying and likely is serving as a testbed.

The two previous secret flights were in roughly 200-plus-mile-high orbits.